The present invention relates to the constitution, production and use of a copolyamide powder which was produced using the following monomer units:
laurolactam or ω-aminoundecanoic acid, and either
dodecanedioic acid or sebacic acid, and either
decanediamine or dodecanediamine,
in shaping processes, and also to mouldings produced through a layer-by-layer process which selectively melts regions of a powder layer, using this specific powder. Once the regions previously melted layer-by-layer have been cooled and solidified, the moulding can be removed from the powder bed.
The selectivity of the layer-by-layer processes here can by way of example be achieved through the application of susceptors, or of absorbers or inhibitors, or through masks, or by way of focussed energy introduction, for example through a laser beam, or by way of glass fibres. The energy is introduced by way of electromagnetic radiation.
A description is given below of some processes which can be used to produce mouldings of the invention, with use, as in the invention, of the copolyamide powder, but there is no intention that the invention be restricted thereto.
A process particularly suitable for the purposes of rapid prototyping is selective laser sintering. In this process, plastics powders in a chamber are selectively and briefly radiated with a laser beam, whereupon the powder particles impacted by the laser beam melt. The molten particles coalesce and rapidly solidify again to give a solid mass. Repeated irradiation of a succession of newly applied layers by this process is a simple and rapid way of producing three-dimensional bodies.
The laser sintering (rapid prototyping) process for producing mouldings from pulverulent polymers is described in detail in the U.S. Pat. No. 6,136,948 and WO 96/06881 (both DTM Corporation). A wide variety of polymers and copolymers is claimed for this application, examples being polyacetate, polypropylene, polyethylene, ionomers and polyamide.
Other processes of good suitability are the SIB process, as described in WO 01/38061, or a process described in EP 1 015 214. Both processes operate with full-surface infrared heating to melt the powder. The first process achievers selectivity of melting by applying an inhibitor, and the second process achieves this through a mask. DE 103 11 438 describes a further process. In this, the energy needed for fusion is introduced via a microwave generator, and the selectivity is achieved through application of a susceptor.
Other suitable processes are those using an absorber, either present within the powder or applied by ink-jet processes, as described in DE 10 2004 012 682.8, DE 10 2004 012 683.6 and DE 10 2004 020 452.7.
Materials that can be used for the rapid prototyping or rapid manufacturing processes mentioned (RP processes or RM processes) are pulverulent substrates, in particular polymers, preferably selected from polyester, polyvinyl chloride, polyacetal, polypropylene, polyethylene, polystyrene, polycarbonate, poly(N-methylmethacrylimide) (PMMI), polymethyl methacrylate (PMMA), ionomer, polyamide, or a mixture thereof.
WO 95/11006 describes a polymer powder suitable for laser sintering which on determination of melting behavior by differential scanning calorimetry at a scanning rate of from 10 to 20° C./min shows no overlap of the melting; and recrystallization peak, and exhibits a degree of crystallinity of from 10 to 90%, likewise determined by DSC, and has a number-average molecular weight Mn of from 30 000 to 500 000, and has a Mw/Mn quotient in the range from 1 to 5.
DE 197 47 309 describes the use of a nylon-12 powder with increased melting point and increased enthalpy of fusion, obtained through reprecipitation of a polyamide previously produced through ring-opening and subsequent polycondensation of laurolactam. This is a nylon-12. An advantage of this precipitated powder is a particle shape advantageous for the application of thin layers, and good separation of melt and powder in shaping processes that construct layers.
EP 03 769 351 describes a single-component polyamide powder with low BET surface area for use in laser sintering. The low BET surface area here is achieved through a downstream mechanical operation, for example in a high-speed mixer.
DE 10 2004 010 160 A1 describes the use of polymer powder with copolymer in shaping processes. This involves thermoplastic random copolymers composed of a very wide variety of monomer units. Monomers are mentioned by way of example for copolyesters, but no details of specific constitutions are given. The MFR value of the copolymers is from 1 to 10 g/10 min. The powders produced therefrom are exclusively obtained by low-temperature milling.
One problem that has to be solved when polymer powder is processed in a mouldless process is that, to avoid what is known as curl, the temperature in the construction chamber has to be maintained with maximum uniformity at a level just below the melting point of the polymeric material. In the case of amorphous polymers, this means a temperature just below the glass transition temperature, and in the case of semicrystalline polymers it means a temperature just below the crystallite melting point. Curl means distortion of the previously melted region, the result being at least some protrusion out of the construction plane. This produces a risk that when the next powder layer is applied, for example via a doctor or a roll, the protruding regions will be displaced or even pulled away completely. The overall construction chamber temperature in the process therefore has to be maintained at a relatively high level, and the volume change brought about by cooling and by crystallization of the mouldings produced by such processes is therefore considerable. Another significant point is that the period required for the cooling process is not inconsiderable, specifically for “rapid” processes.
An intrinsic feature of semicrystalline thermoplastics which is difficult to control in many instances is their crystallinity, or the resultant volume change during cooling of the melt. Although the volume change caused by crystallinity in a single layer can substantially be compensated by using a very complicated and precise temperature profile, the volume change resulting from crystallization in three-dimensional mouldings of any desired structure is non-uniform across the moulding. By way of example, formation of crystalline structures depends on the cooling rate of the moulding, and the rate at locations of varying thickness or at locations involving angles differs from the rate at other locations within the moulding. Secondly, high enthalpy of fusion is needed for good delineation between the molten regions and their environment, this being a deciding factor in relation to the dimensional accuracy of the component.
In terms of processing reliability, another disadvantage of the commercially available and most widely used homopolyamide, nylon-12 powder, is a relatively high BET surface area in comparison with ground powder or powder obtained by polymerization in solution. The high BET surface area leads to impaired powder flowability, and although this can be countered by addition of a suitable powder-flow aid, this is achieved at the cost of decreased processing latitude, and the result, less reliable processes, is rather counterproductive for rapid manufacturing. The high BET surface area also has a disadvantageous effect on the warpage of the components. Powders obtained by milling with low BET surface area, for example nylon-11, which is likewise used commercially, have sharp-edged particles, the shape of which is likewise disadvantageous for processing reliability.
A disadvantage of amorphous thermoplastics is high viscosity, permitting coalescence only markedly above the melting point or the glass transition temperature. Mouldings produced by above processes using amorphous thermoplastics are therefore often relatively porous; sinter necks are merely formed, and the individual powder particles are discernible in the moulding. However, if energy input is increased in order to reduce viscosity there is the additional problem of dimensional accuracy; by way of example, heat conducted from the regions to be melted into the surrounding regions makes the contours of the moulding indistinct.
There is therefore a need for a precisely adjustable mixture of the properties which were described in the introduction and are typical of amorphous and of semicrystalline polymers. Unfortunately, copolymers of the background art do not satisfy these requirements.
A disadvantage of the copolymers described in the literature hitherto, in particular copolyamides for use in mouldless shaping processes, is that although the composition can achieve a lowering of the melting points, with a favourable effect on processability and on shrinkage, specifically by using at least one aromatic monomer unit, the result of this is reduced crystallinity, and the crystallite melting point therefore does not then describe the transition from solid to liquid, but increasingly describes the glass transition, this transition being gradual and dependent on the compositions of the copolymers. The aromatic monomer unit can by way of example be terephthalic acid or isophthalic acid. However, these aromatic components generate a counter-effect of markedly increasing the viscosity of the melt, making coalescence of the powder particles more difficult. A compromise therefore always has to be found between competing targeted properties. Furthermore, powder composed of the usual copolymers cannot be precipitated and therefore has to be obtained by other processes, such as low-temperature milling. However, this gives sharp-edged particles which have a disadvantageous effect during processing.
It was therefore an object of the present invention to permit, in mouldless shaping processes, the use of a polymer powder which combines the contradictory properties of low BET surface area, round grain shape and low viscosity just above the crystallite melting point, while simultaneously having high enthalpy of fusion. The process here is a layer-by-layer process in which regions of the respective powder layer are selectively melted by electromagnetic energy and, after cooling, have bonded to give the desired moulding.